This invention relates to a process for the recovery of iron from coal fly ash. More specifically, this invention relates to a process for recovering iron oxide, as a concentrate suitable for reduction, from pulverized coal fly ash.
Fly ash is being produced in increasing quantities as the result of the expanding use of pulverized coal to produce electricity. Fly ash is formed from the inorganic constituents present in the coal, and is collected from the flue gases by mechanical separation and/or electrostatic precipitation. The primary constituents of the ash are the oxides of silicon, aluminum, iron and calcium. In addition, trace elements of selenium, arsenic and other heavy metals are present. These trace components, some of which are toxic, cause environmental problems in the long term disposal of the ash.
The treatment of fly ash to recover its constituent metals has been proposed as an alternative to its disposal. Alumina is the primary product although titanium, iron, and some of the trace metals may also be recovered. Some of the proposed processes produce an iron-rich fraction from a pretreatment step to remove iron by magnetic separation of the ash. Direct uses for the magnetic fraction include heavy media material for coal washing and filler for high density cement. In addition, its use as an iron ore has been suggested, but attempts to market the material have not been successful.
About 10 to 15 weight percent of the fly ash resulting from the combustion of pulverized eastern and midwestern bituminous coal has magnetic properties which allow separation from the nonmagnetic fly ash through the use of various magnetic separation devices. In general, 60 to 70 weight percent of this magnetic fraction is Fe.sub.2 O.sub.3, the remainder consisting of oxides of aluminum, silicon and other heavy metals.
Processes for the recovery of iron and aluminum have generally consisted of various methods of chlorination to produce metal chlorides which are then separated and reduced to obtain the metal. One such process is described in U.S. Pat. No. 4,159,310, which was issued June 26, 1979 to Reynolds et al.
In U.S. Pat. No. 4,130,627, fly ash is treated with a solution at pH 11-14 and filtered solids are then treated with HCl or HF to form a solution of chlorides or fluorides of iron and aluminum. The iron is then recovered by electrodeposition at pH 1-3.
Treatment of the magnetic portion of the fly ash with only a strong alkali leach, such as 30 weight percent NaOH and at low fly ash to alkali ratios such as 50 to 70 grams ash per liter of solution has not proven successful in beneficiating the iron oxide due to the limited solubility of the oxides of silicon and aluminum in the caustic solution. Furthermore, the economic factors involved in the use of many of these chemical compounds, particular the caustics, makes many beneficiation or recovery processes unfeasible.